The present invention relates to a process for the control of the birefringence properties in waveguides.
It is known to construct optical waveguide devices having internal waveguide structures. These devices can be fabricated utilizing plasma enhanced chemical vapour deposition processes on a silicon substrate. FIG. 1 illustrates such a layer 1 formed on a substrate 2 including a waveguide 3 therein.
Often non-symmetrical birefringence effects will result from the formation process. The first birefringent effect denoted xcex2form will be due to the circumference characteristics of the waveguide 3. The second effect denoted xcex2stress will be due to several stresses associated with the thermal coefficient mismatch of the substrate 2 and deposited layer 1.
There is therefore a need to alter the birefringence properties of such waveguides to nullify the birefringence should this be required.
On the other hand, it may alternatively be desirable to alter birefringence in other internal waveguide structures, for example in the core of an optical fibre. The alteration must not necessarily be with the view to nullify the birefringence, but could for example be with the view to introduce birefringence.
The present invention provides a method of modifying birefringence properties of an optical waveguide formed within a carrier, comprising:
inducing a stress anisotropy in the waveguide;
exposing the waveguide to an electromagnetic radiation selected to induce refractive index changes of first and second polarisation modes of the waveguide as a function of the fluence of radiation exposure;
terminating the radiation exposure at a fluence for which the refractive index of the first polarisation mode decreases in response to increasing fluence and the refractive index of the second polarisation mode increases in response to increasing fluence.
It has been found by the applicant that stress-sensitive transitions in a waveguide can be utilised to alter the birefringence properties of the waveguide.
The step of inducing the stress anisotropy in the waveguide may be performed as an independent pre-stressing step.
The radiation may comprise UV radiation.
The carrier may comprise a planar structure in which the waveguide is buried.
Alternatively, the carrier may comprise a cladding of an optical fibre, the waveguide comprising the core of the optical fibre.
The waveguide may be formed from a silica-based material.
Where the stress anisotropy is already present in the waveguide, the invention may alternatively be defined as a method of modifying birefringence properties of an optical waveguide formed within a carrier, comprising:
exposing the waveguide to an electromagnetic radiation selected to induce refractive index changes of first and second polarisation modes of the waveguide as a function of the fluence of radiation exposure;
terminating the radiation exposure at a fluence for which the refractive index of the first polarisation mode decreases in response to increasing fluence and the refractive index of the second polarisation mode increases in response to increasing fluence.